If you’re just now getting back into a workout routine after a few weeks off for the holidays, or after injuries or laziness kept you grounded for a long time, don’t feel guilty, and don’t worry. You will get that strength and fitness back. Compared to somebody who’s never trained, your experience gives you a huge advantage that’ll make it easier to get back to form.

Don’t forget your first victory: That you came back at all! If you’re having trouble with the mental side of getting back in the game after a workout break, remember our five step guide to returning to the gym:

Don’t beat yourself up

Evaluate your losses

Be thankful for how far you’ve come

Make a plan

Execute that plan

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Let’s peek under the hood of that plan. We spoke with Exercise Physiologist Walter Thompson of Georgia State University to learn what happens to your body during that time off, and what you can expect when you get back in the saddle. The good news is that even if it doesn’t feel like it now, you’ll get back to your stronger, faster self soon—if you’re careful to avoid injury.

How Much Did My Break Set Me Back?

Lots of different body systems change with exercise, and can change back when you become sedentary again. The amount of blood in your body, which increases when you’re fit, is one of the first things to decrease. Runners use a measure called VO2max, or how quickly they find themselves out of breath, as a measure of fitness. After even two weeks off, their VO2max begins to drop, and they’re out of breath sooner than before. After two months, you’ll have lost about 15% of it, according to one classic study published in the Journal of Applied Physiology. If you were an athlete for years, even after three months you’d still be better off than somebody who’s never trained. But this doesn’t apply to brand-new gains, like if you did a couch-to-5k program and then went straight back to the couch. You’ll have to start over.

Some other things stick around even longer. The extra capillaries you’ve grown to supply blood to your muscles are still there after a year. Your heart will still be strong, and your lungs will still have a greater capacity than before you started training.

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As for how much fitness you lost, there isn’t a formula that will tell you precisely; these numbers are just guides based on what the subjects of a few studies experienced. So to summarize:

If you took a few weeks off: you’ll have a small dent in your cardio fitness, and lost next to none of your strength.

If you took a year off (but were in good shape before that): you’ll have lost at least 15% of your cardio fitness, and about half of your strength.

If you took many years off (say, you were a college athlete during the Clinton presidency): assume you’re starting from scratch. You may be able to make some gains faster than if you were a true newbie, but let that be a pleasant surprise.

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Before you make your comeback plan, it’s important to look at why you took that break, and what happened in the meantime. For example, if you gained weight, the extra pounds might make it harder to run your old times or do the same number of pull-ups you used to do.

If you quit because of injury, you’ll want to make sure you fully addressed that injury. If not, now is a good time to visit a doctor. Find out what’s wrong and get it fixed, and see a physical therapist to correct any muscle imbalances or weaknesses that were either a cause or a result of the injury.

How Quickly Will I Get It Back?

If your break was only a few weeks long—just over the holidays, let’s say—you might only need to take it easy for a few sessions before you’re back up to speed.

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What if you’ve been on break for a year or more? Like me—I started 2015 recovering from knee surgery, and ended it with a new baby and a busy work schedule. It feels like ages since I’ve really challenged myself in the gym. Thompson has good news:

What I generally say is, look, you took a year off. It’s not going to take a year to get back to where you were, but you’re not where you were a year ago. So back off some, I generally say between a third and half of the weight that you lifted, and then take a week or two to get back into your regular routine. There will be adaptation and it’ll be pretty quick.

That doesn’t mean you’ll be lifting your old personal bests after a week or two, but you’ll be well along the way. A realistic timeframe, Thompson says, is to be back to normal within about two months. That goes for both strength and cardio.

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So if you normally bench 200 pounds, don’t expect to do much more than 100 your first day back. If you had just worked up to full push-ups, you may have to go back to your modified versions.

For an endurance sport like running, you still want to reduce the intensity of your workout. Thompson suggests walking at first, then sprinkling in jogging intervals until you’re able to jog your full workout at your old speed. The exact distance doesn’t matter. Runners like to obsess over mileage, and often cite 10 percent as the perfect week-to-week increase. That’s just a rule of thumb, though, and it’s possible to make progress without it. Thompson agrees that it’s arbitrary, writing in an email: “There is no scientific (or medical) proof of the 10% rule.”

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After two months, if you’re not back to where you were, your workout program likely needs to be tweaked—especially if you’ve been doing whatever you always did, but your goals have changed. Find a good trainer and make sure that your program is challenging enough and geared towards your new goals.

What Should I Do the Next Time I Have to Take a Break?

Life happens. Even if you swear you’ll never take time off again, someday you’ll travel, or get injured, or take on an exciting new project that eats up all your free time.

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The key rule is don’t stop completely. Even if you can’t do your regular workout, find something you can do. Stationary bikes and treadmills are an easy go-to when the weather is bad, and Thompson recommends rowing machines for a change of pace (it seems there’s one tucked away in the corner of every gym). Other options might include quick workouts, for example from the Sworkit app (if your obstacle is lack of time) or exercises you can do at home or in a hotel, if access to equipment or outdoor spaces is the issue.